In general, large-size synthetic quartz glass substrates are manufactured by lapping a plate-shaped synthetic quartz stock with a slurry of loose abrasives such as alumina in water for abrading away raised and recessed portions (irregularities) and polishing with a slurry of abrasives such as cerium oxide in water.
Double and single-side lapping machines used in the lapping process are designed so as to force a substrate against a lapping plate and utilize a reaction force against the resultant elastic deformation for flatness correction. They have the drawback that as the substrate size becomes larger, the reaction force considerably decreases, leading to a reduction of the ability to remove moderate irregularities on the substrate surface.
The problem may be solved by measuring a flatness and parallelism of a large-size substrate and partially removing raised portions and thick portions of the substrate on the basis of the measured data, as disclosed in JP-A 2003-292346. Also a quartz glass substrate may be polished by immersing the substrate in a polishing fluid, adjusting the attitude of the substrate, and bubbling a gas into the polishing fluid, the gas bubbling step being based on data indicative of the relationship of flatness and flaws of the substrate to positions on its surface, as disclosed in JP-A 2007-001003. A quartz glass substrate with a high flatness is produced at a high processing stability. However, as the substrate size becomes larger, the working time is prolonged. There exists a need for an economical method capable of flatness correction within a short time.